Finnish Pat. No. 610033 discloses a process for the manufacture of an alkali-soluble cellulose compound without using carbon disulfide or any other environmentally harmful chemicals. In this method cellulose is heated with urea in an organic liquid in which urea is substantially insoluble.
U.S. Pat. No. 4,404,369 describes an improvement in this method according to which cellulose is treated with liquid ammonia having urea dissolved therein, the ammonia is removed, resulting in cellulose having urea distributed therethrough, and the cellulose-urea is heated to cause reaction between the urea and the cellulose to form the alkali-soluble cellulose carbamate which can be precipitated from alkali solution in the form of cellulose carbamate film or fiber.
The above procedures are based on the fact that when urea is heated to the melting point, or higher, it begins to decompose into isocyanic acid and ammonia. Isocyanic acid is not a particularly stable compound and tends to become trimerized into isocyanuric acid. The isocyanic acid also tends to react with urea to form biuret. Still further, the isocyanic acid reacts with cellulose to produce the alkali-soluble cellulose derivative cellulose carbamate.
The reaction may be written as follows: ##STR1##
The thus produced cellulose compound, namely cellulose carbamate, may be dried after washing and stored even for prolonged periods of time, or it may be dissolved, for instance for fiber manufacture, in an aqueous alkali solution. Cellulose carbamate fibers or films may be manufactured from such solution by spinning or by extruding, in the same manner as in the viscose manufacturing process. The stability of cellulose carbamate and the feasibility of its transport in dry state afford a great advantage compared with cellulose xanthate in the viscose method since the latter cannot be stored or transported, neither in dry nor in solution form.
When it is desired to manufacture continuous fiber or filament suitable, for example, for textile use of the like, from the cellulose carbamate, the cellulose carbamate is first dissolved in an alkali, e.g. sodium hydroxide solution. Cellulose carbamate may be spun through orifices from the solution into an acid precipitation bath which causes precipitation of the cellulose carbamate. Film formation can likewise be accomplished by extruding the solution into an acid precipitation bath. The precipitation may also be accomplished by spinning or extruding into lower alcohols such as methanol, ethanol or butanol, or into hot aqueous salt solutions.
The spinability of the alkali solution of cellulose carbamate is influenced by several factors, including: carbamate content, viscosity and clogging number of the solution. The clogging number is the amount of gel-resembling, only partly dissolved, fiber particles contained in the solution. As the carbamate content of the solution is increased, the fiber spinning capacity is increased. However, the increase in carbamate content in the solution results in an increase in the viscosity of the solution, which in turn restricts the fiber spinning capacity. This even occurs when zinc salts are used as an agent for improving the solubility of the carbamate.